It is known in the art to provide a thermostatic mixing valve to control the temperature of warm water by controlling the proportions of supplied hot and cold water.
From the time that piped hot and cold running water became a common feature of architecture there has been an awareness of the occasional inconvenience resulting from a sudden change in temperature or pressure to the hot or cold water supplied to a common outlet such as a shower. In extreme cases, the resulting change in temperature of the water from an outlet can result in substantial discomfort or injury to the user, most especially in a shower cubicle. A sudden pressure drop in the cold water supply, such as occurs when an adjacent toilet is flushed, will result in a sudden increase in the temperature of the shower water and a potentially dangerous situation.
Self-regulating mixing valves for mixing hot and cold water can be defined according to the following six categories:
The first type of self-regulating valve utilizes a pressure-balancing mechanism to prevent the sudden temperature changes that arise from pressure changes in one of the supply lines. This type of valve, however, will not respond to a decrease in the temperature of water in the hot water supply line. It will therefore not compensate for a gradual temperature change that occurs as the hot water tank cools off due to peak household demand or as the household demand is subsequently reduced. This type of mixing valve is typified by the valves disclosed in U.S. Pat. No. 2,308,127 to Symmons on Jan. 12, 1943 entitled “Non-scald Mixing Valve” and U.S. Pat. No. 6,050,285 to Goncze, et al. on Apr. 18, 2000, entitled “Pressure Balancing Valve.”
Similarly, as disclosed in U.S. Pat. No. 5,161,737, pressure-reducing valves equalize pressures in both inlets to the mixing valve, which then stabilizes the temperature. The design of such pressure equalizing valves is complex and these are still affected by any temperature changes of the incoming fluids. Consequently, these pressure equalizing valves cannot maintain a stable temperature.
Another type of self-regulating valve provides a shut-off mechanism to automatically stop or divert the water flow discharged from the valve when the temperature of the outlet water exceeds some preselected maximum temperature. An example of this type of valve is disclosed in U.S. Pat. No. 2,534,378 to Schlaich on Dec. 19, 1950 entitled “Safety Control For Shower Heads And Other Hot Water Outlets.”
A third type of self-regulating valve is a directly driven thermostatically controlled valve. These valves typically comprise a housing having a mixing chamber, hot and cold water inlets and a proportioning valve disposed between the inlets and the mixing chamber. A temperature responsive element, disposed within the mixing chamber, is coupled directly to the proportioning valve at one end and to the housing at the other end. Examples of this type of valve are disclosed in U.S. Pat. No. 2,272,403 to Fields on Jun. 10, 1939, entitled “Mixing Valve,” U.S. Pat. No. 2,383,215 to Reynolds on Jul. 26, 1943, entitled “Mixing Valve,” U.S. Pat. No. 2,463,640 to Plett on Mar. 8, 1949, entitled “Thermostatic Water Control” and U.S. Pat. No. 3,539,099 to Grohe on Nov. 10, 1970, entitled “Thermostat Controlled Mixing Faucet.”
In a thermostatic mixing valve, which is another version of the above mentioned directly-driven thermostatically-controlled valve, such as those disclosed in U.S. Pat. No. 5,108,032 to Stewart on Apr. 28, 1992 entitled “Fluid Mixture Control Valve,” U.S. Pat. No. 5,110,044 to Bergmann on May 5, 1992 entitled “Sanitary Mixing Valve” and U.S. Pat. No. 5,203,496 to Kline on Apr. 20, 1993 entitled “Thermostatic Control Valve with Fluid Mixing,” the relative hot and cold fluid flow rates are controlled by a proportioning valve set directly in accordance with a thermally responsive element.
However, such directly driven thermostatically controlled valves fail to provide a constant outlet water temperature but, instead, greatly reduce the deviation of the outlet water temperature from a preselected temperature consequent to temperature or pressure changes in the supply lines. The user selects a temperature by adjusting the proportioning valve position to give a preselected temperature. If the water supply temperature and pressure remain constant, the proportioning valve remains stationary and the outlet water remains at the preselected temperature. The dynamic system consisting of the temperature responsive mechanism, connected directly to the proportioning valve at one end and to the housing at the other end, remains in static equilibrium.
If, however, the pressure or the temperature of the water in one of the supply lines assumes a new value, the temperature of the outlet water is temporarily changed. The temperature responsive mechanism responds to the temperature change by directly moving the proportioning valve in the direction that will tend to restore the mixed water temperature to its previous level. As the proportioning valve responds the outlet water temperature changes and eventually causes the temperature responsive mechanism to reverse the direction of its movement, giving rise to a period of oscillations. Thereafter the dynamic system will seek a new equilibrium position corresponding to a new equilibrium outlet water temperature. This new equilibrium water temperature is not identical to the preselected temperature since the position of the temperature responsive mechanism corresponding to the preselected temperature is the initial mechanism position.
The fourth type of self-regulating valve disclosed is the feedback servomechanism valve. This valve uses a valve element not directly linked to the housing by the temperature responsive element. When the temperature responsive element senses a temperature deviation from a preselected value, a signal is transmitted to a valve element causing movement in a direction to restore the outlet temperature. When the preselected outlet temperature is reached as sensed by the temperature responsive element, signal transmission ceases. Examples of this type of valve are disclosed in U.S. Pat. No. 1,869,663 to Cartier on Aug. 2, 1932, entitled “Thermostatic Mixing Means,” U.S. Pat. No. 2,449,766 to Brown on Sep. 21, 1948, entitled “Means for Producing Uniform Fluid Mixtures,” U.S. Pat. No. 2,542,273 to Brown on Feb. 20, 1951, entitled “Temperature Controlled Mixing Valve,” U.S. Pat. No. 2,550,907 to Brown on May 1, 1951, entitled “Temperature Controlled Mixing Valve,” U.S. Pat. No. 3,561,481 to Taplin on Feb. 9, 1971, entitled “Fail-Safe Servo-Controlled Mixing Valve,” U.S. Pat. No. 3,642,199 to Halkema on Feb. 15, 1972, entitled “Thermostatic Mixer for Hot and Cold Liquids” and U.S. Pat. No. 4,458,839 to MacDonald on Jul. 10, 1984, entitled “Thermostatic Valve Assembly.” All the above-mentioned inventions include the use of a small portion of the flow to regulate valve element movement in accordance with temperature change.
The servomechanism valves represent an improvement, in theory, over directly operated valves because the temperature responsive element is restored to the same equilibrium position when the preselected temperature is reached, regardless of the temperature or the pressure of the supply water. Since no oscillation fading period is required to reach an equilibrium position, as with the directly operated thermostatically controlled valve, servomechanism valves respond more quickly to adjust the outlet water temperature. Consequently, servomechanism thermostatic valves more accurately maintain the preselected temperature.
It should be noted that most servomechanism valves do not respond in the theoretical fashion described above given a hot and cold inlet water extreme pressure imbalance because the imbalance alters the equilibrium position of the valve member.
Furthermore, the disclosed servomechanism valves are extremely large compared to conventional valves. Also, common to servomechanism valves and two stage valve assemblies, the long narrow fluid passageways are easily clogged by suspended particles in the water supply.
The fifth type of valve is a two-stage valve assembly. A first stage, comprising pressure equalization means, compensates for pressure changes, maintaining a constant hot and cold-water pressure ratio. Downstream, a second stage is a thermostatically controlled proportioning valve. An example of this type of valve is disclosed in U.S. Pat. No. 3,539,099 to Grohe on Nov. 10, 1970, entitled “Thermostat Controlled Mixing Faucet” Utilizing this type of valve, the outlet water temperature remains constant over a wide range of supply pressures and temperatures. The major disadvantage of this valve is a substantial increase in the number of components, the cost of assembly, and the space required for the valve assembly compared to one-stage valves.
A sixth approach to temperature control is to employ the use of a degenerative feedback device, usually comprising hot and cold water inlets, a mixing chamber, a stepper or other motor controlled valve, a temperature sensor, an electric comparator unit for comparing the temperature sensor signal with a reference signal, and a motor controller for keeping the signal differences as low as possible. Such systems often include sophisticated electronics, a microprocessor and an electrical power supply. Safety protection against electrical shock is needed, as well as protection against power loss to avoid the risk of losing control of the mixed water temperature. These devices are expensive and not applicable, for example, to a domestic shower.
Referring now to U.S. Pat. No. 5,427,312 to Simonov, et al. on Jun. 27, 1995, entitled “Thermostatic Mixing Valve and Method of Use Thereof,” therein is disclosed a thermostatic mixing valve and method of use thereof. The thermostatic mixing valve, having incoming hot and cold water mixed in a mixing chamber, is controlled by a thermally responsive element disposed in the fluid outlet from the mixing chamber. A distributor for distributing the operative flow is controlled by the thermally responsive element. Activated by pressure of the fluid, a drive is connected to the distributor for controlling the position of the drive. The distributor is used to control the hot and cold fluid flows. Channels are provided to connect the distributor with one of the fluid inlets, to connect the distributor with the outside of the valve body, to conduct away used operating fluid and to connect the distributor with the drive.
If the pressure or temperature of the supply of either hot or cold fluid to the mixing valve disclosed in U.S. Pat. No. 5,427,312 changes to a new value, the temperature of the outlet stream will temporarily change until the proportioning valve restores the outlet temperature. There is a significant drawback to this mixing valve. Only a small proportion of the flow of one of the incoming fluids is utilized as an operative flow, which must then be separately discharged from the valve body. This operative flow is carried in a small-bore conduit, as a result of which there is a substantial risk of blockage of the small-bore conduit.
Thus there is a need to provide a mixing valve which is inexpensive, compact and provides a stable out-flow temperature in spite of variations in temperature or pressure of the hot and cold supply liquids. There is also a need for such a valve to protect against flow stoppage of either the hot or cold supply, so as to avoid exposing a user to the risk, most especially from the supply of the hot liquid stream alone. In addition, the mixing valve must not be subject to failure or malfunction as a result of accumulation of solid particles from the supply streams or of a power failure.